Plasma is generally defined as a collection of charged particles containing about equal numbers of positive ions and electrons and exhibiting some properties of a gas, but differing from a gas in that plasma is generally a good conductor of electricity and may be influenced by a magnetic field. A plasma may be generated, for example, by passing a gas through an electric arc. The electric arc will rapidly heat the gas by resistive and radiative heating to very high temperatures within microseconds of the gas passing through the arc. Essentially any gas may be used to produce a plasma in such a manner. Thus, inert or neutral gases (e.g., argon, helium, neon or nitrogen) may be used, reductive gases (e.g., hydrogen, methane, ammonia or carbon monoxide) may be used, or oxidative gases (e.g., oxygen, water vapor, chlorine, or carbon dioxide) may be used depending on the process in which the plasma is to be utilized.
Plasma generators, including those used in conjunction with, for example, plasma torches, plasma jets and plasma arc reactors, generally create an electric discharge in a working gas to create the plasma. Plasma generators have been formed as direct current (DC) generators, alternating current (AC) plasma generators, as radio frequency (RF) plasma generators and as microwave (MW) plasma generators. Plasmas generated with RF or MW sources may be referred to as “inductively coupled” plasmas. In one example of an RF-type plasma generator, the generator includes an RF source and an induction coil surrounding a working gas. The RF signal sent from the source to the induction coil results in the ionization of the working gas by induction coupling to produce a plasma. In contrast, DC- and AC-type generators may include two or more electrodes (e.g., an anode and a cathode) with a voltage differential defined therebetween. An arc may be formed between the electrodes to heat and ionize the surrounding gas such that the gas obtains a plasma state. The resulting plasma, regardless of how it was produced, may then be used for a specified process application.
In some applications, plasma reactors may be used for the high-temperature heating of material compounds to accommodate the chemical or material processing thereof. Such chemical and material processing may include the reduction and decomposition of hazardous materials. In other applications, plasma reactors have been utilized to assist in the extraction of a desired material, such as a metal or metal alloy, from a compound which contains the desired material.
As will be recognized by one of ordinary skill in the art, the creation of a plasma may require significant electrical power. Consequently, the use of plasmas in the production of various materials in commercial quantities is somewhat restricted in view of the costs attendant to purchasing the electricity and equipment necessary to produce the plasma and the other equipment to produce the product of interest.
Additionally, for certain chemical processes, chemical flame burners are used to combust a reactant for purposes of reacting it with another material in order to produce a resulting compound. These conventional flame burners, which are utilized in the industry, consume a significant amount of fuel, and air, to maintain the high operational temperatures that are necessary for these chemical reactions to occur.
Combustion flame-plasma hybrid reactors, such as described in U.S. Pat. No. 7,354,561 assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein in its entirety by reference, may utilize both a combustion flame and a plasma to facilitate the chemical conversion of a reactant to a product.
Although combustion flame-plasma hybrid reactors, such as those disclosed in U.S. Pat. No. 7,354,561, have been successful for uses such as the conversion of sodium borate to sodium borohydride, a chemical that is useful as a reducing agent in chemical and pharmaceutical processing and is a chemical of great interest for new energy storage and fuel cell applications, several technical issues remain unresolved. Additionally, several modifications and improvements to combustion flame-plasma hybrid reactors have also suffered from technical issues. For example, for some combustion flame-plasma hybrid reactors, the combustion flame cannot be established in the arc channel and it must be established externally first and then inserted into the arc channel. Also, significant water cooling of the modular plasma torch unit may be required and may result in a lower thermal efficiency for the system.
Additionally, the modular electrodes and the arc channel may be water-cooled components that may collect condensed water vapor, which may form from the combustion process. Condensed water on the electrode surfaces may create significant difficulties for arc ignition in the plasma unit, as it may increase the electrode break down potential significantly. In other words, a much higher voltage may be required in order to produce and maintain an arc between the electrodes when condensed water is present on the electrode surfaces. In view of this, only power supplies with very high voltages may be able to break down the electrodes with high artificial potential barriers and initiate an electric arc between the electrodes.
In order to use a normal voltage power supply, a temporary solution to the problem of water condensation on the electrodes may be shielding the water-cooled metal electrode surfaces with graphite inserts, as graphite is a conductive refractory material that may allow the surface of the electrode to exceed 100° C., which may prevent the condensation of water vapor thereon. Additionally, low ionization potential materials, such as sodium (Na), may be required to ignite the arc. However, the graphite inserts may react with combustion products, such as carbon dioxide (CO2) and water (H2O), and may be continuously consumed during operation. Furthermore, as the graphite inserts are consumed, the metal electrodes, such as tungsten alloy electrodes, may become exposed and the metal electrodes may also react with combustion products, such as CO2 and H2O, and be consumed. This may result in an unstable arc operation.
Another technical issue with combustion flame-plasma hybrid reactors involves the arc channel used to confine the arc column. For example, a sodium (Na) species in the arc may dissolve in the quartz of a water-cooled quartz tube that may form the arc channel, which may form a layer of soda glass on the tube surface. As a result, a material mismatch between the surface of the soda glass layer and the underlying quartz may cause the arc channel to crack and cooling water may leak into the arc channel and disrupt the arc.
In view of the foregoing, which should not be construed as admitted prior art, it would be advantageous to provide improved reactors, such as combustion-flame hybrid reactors, and related methods, devices and systems that address shortcomings in the art.